The present invention relates to a heat roll fixing unit for fixing toner on a surface of a recording sheet employed in an image formation device, such as a printer, for forming a visible image by a so-called electrophotographic system. More particularly, the present invention relates to a structure of the heat roll fixing unit in which it is possible to prevent the recording sheet from being caught around a roller and thus being poorly fed through the printer.
Conventionally, there has been known a so-called electrophotographic system, wherein a latent image is formed by exposing a photoreceptor on a surface of a photoconductive drum, adhering toner to the latent image to change the latent image to a visible toner image, and transferring the toner image to a recording sheet and fixing the image to the recording sheet at a fixing unit.
An example of an image formation device using this electrophotographic system is an electrophotographic printer for forming an image on a continuous-form recording sheet similar to that used in a conventional line printer and the like.
The continuous-form recording sheet, similar to that used in the line printer, is a folded sheet provided with a perforated tear line, referred to as a fan-folded continuous-form sheet (hereinafter, simply referred to as a continuous-sheet), and the folded portion thereof can be easily torn off along the perforated tear line.
Usually, in a image formation device using the conventional electrophotographic system, such as a copy machine and the like, a so-called heat roll fixing method is often used as a method of fixing toner, having been transferred on the recording sheet in accordance with the latent image, by which the recording sheet is pressed against a heated roller and toner is heated and fusedly adhered to the recording sheet by the heat applied to the roller.
To effect the heat roll fixing, a fixing roller pair is composed of the heat roller heated to high temperatures and a press roller, which are disposed in parallel to each other with the recording sheet held therebetween to cause the recording sheet to be pressed against the heat roller for heating. The heat roll fixing method is advantageous in that the fixing can be carried out at high speed with a high thermal efficiency.
When the heat roll fixing method is employed in the electrophotographic printer using the above continuous sheet as the recording sheet, a drawback arises in that when the continuous-sheet is held between the heat roller and the press roller when printing is not executed, the same portion of the recording sheet is burned or blistered by the heat applied from the heat roller. To overcome this drawback, a structure of the rollers, in which one roller can be retracted to prevent the recording sheet from coming into contact with a heat roller when printing operation is not executed, has been proposed in, for example, Japanese Patent Provisional Publication No. HEI 1-163790.
Nevertheless, even if the heat roller is, for example, rockably retracted when printing is not executed, a problem arises in that the continuous-sheet may be bent to a direction in which the heat roller is retracted and may be continuously in contact with the heat roller with the result that such drawbacks as burning and blistering occurring. Further, a drawback also arises in that the continuous-sheet is adhered to the heat roller by the depositing force of the toner and cannot be separated, therefrom and thus the continuous-sheet may be caught around the roller and cannot be fed, i.e., a so-called jam may occur.